1. Field of the Invention
This invention relates to a photographic electrostatic process of making printing plates useful in the lithographic printing process, and to hydrophilizing compositions useful in this process.
2. Description of the Prior Art
Processes for producing lithographic printing plates by means of the electrophotographic process are well known in the art. The common way of producing such a printing plate is to first form an electrostatic plate having an oleophilic toner image deposited on a photoconductive layer such as zinc oxide deposited in an insulating resin binder by means of the electrophotographic process well known in the art. This thus formed electrophotographic plate is then converted into a lithographic printing plate by means of a hydrophilizing composition which is applied to the plate to convert the areas of the plate not containing the toner image to a more hydrophilic state. Numerous conversion baths have been disclosed in the prior art for such processes. See U.S. Pat. Nos. 3,573,041; 3,617,266; 3,001,872; 3,107,169; and 3,323,451, each of which is incorporated herein by reference. The most widely utilized commercial conversion wash is one containing ferrocyanide. Because of the toxic by-products this solution has serious disadvantages. This becomes especially critical as government regulations increasingly prohibit or restrict the use of such toxic materials.
A second lithographic printing plate which is in wide commercial use is the multilayer silver halide diffusion transfer printing plates described in U.S. Pat. No. 3,146,104, incorporated herein by reference. This plate is a camera speed printing plate which gives a somewhat higher quality print on the lithographic printing press than the electrostatic printing plate described above. Also the diffusion transfer printing plate has a somewhat longer run length generally than that of the electrostatic printing plate. Thus the diffusion transfer printing plate often is used for run lengths of 5,000 to 10,000 copies whereas an electrostatic printing plate generally has a run length of no greater than about 200 to 2,000 copies before serious degradation of the image or background occurs.
Many times in in-plant printing facilities and in instant printing shops and the like, it is desirable to make both kinds of printing plates described above depending upon the quality and quantity of print desired by the customer. However, at the present time, one of the serious problems is that the printing plates of the two processes are not compatible on the same press under the same printing conditions. Thus a different fountain solution is needed for each of the two types of printing plates. Thus in order to eliminate the requirement of a separate printing press for each type of printing plate or to eliminate the need to undergo time consuming clean-up operations on the press to change from one type of printing plate to the other, it is desirable to provide an electrostatic lithographic printing plate which is compatible on the same press under the same conditions with the silver halide diffusion transfer plate described above.